Higher Chemistry Intermolecular Forces NEW LEARNING Grouping elements by bonding and structure – focus on Titanium Grouping compounds by bonding and structure. Periodic trends REVISION Elements are the simplest substances and consist of one type of atom. They are arranged in the Periodic table in increasing atomic number.

Starter Task Complete Quick Test 4 in your booklet and we will review as a class. You have 10minutes! S3 Revision

Answers . Quick Test 4 C 2. C 3a. Ionic 3b. Ionic 3c. Covalent 3d. Covalent 3e. Ionic 3f. Covalent 4a. D 4b. G 4c. B 4d. F 4e. E 4f. D 4g. F S3 Revision

Lesson 1: Intermolecular Forces Today we will learn to Describe the forces which exist BETWEEN atoms or molecules in a substance. We will do this by Considering the effect of these forces on B. Pt, density and viscosity of materials. We will have succeeded if We can describe at least 3 different types of intermolecular attraction.

Intermolecular Forces The noble gases are elements which exist as single atoms and as they are gases their atoms move around very quickly and have no attraction for one another. However, if you lower the temperature enough they will eventually turn into liquids. As a liquid the atoms are beginning to join up in groups and stay together, there must, therefore, be some sort of attraction between the atoms. These attractions are known as Van der Waal ‘s forces. You notice that as the mass of the atoms increases so does the temperature that they boil at. In other words the bigger the atom the more energy it takes to break the Van der Waal forces keeping the atoms together as a liquid.

Intermolecular Forces It is thought that Van der Waal forces occur when the electrons in atoms move randomly around forming slight dipoles (where one side of the atom is positive and the other negative), these slight dipoles can cause dipoles in other atoms and a brief attraction occurs but then breaks as the electrons continue to move. The more electrons there are the bigger the potential dipole and the stronger the Van der Waal force. Bigger atoms also require more energy to move and so more energy is required to get the atoms moving fast enough to break the Van der Waal forces between them. Covalent molecules will also exhibit Van der Waal forces between their molecules.

Bonding Between Molecules There are attractive forces between covalent and polar covalent molecules which can affect their properties. These attractions between molecules are called Van der Waals or intermolecular forces (or bonds). (intramolecular = within the molecule, e.g. covalent bond) There are 3 types: London Dispersion Forces Dipole-dipole Attractions (permanent dipole-permanent dipole) Hydrogen Bonds are a special type of dipole-dipole attraction which is particularly strong.

London Dispersion Forces This is the weakest form of intermolecular bonding and it exists between all atoms and molecules. Dispersion forces are caused by uneven distributions of electrons. The atom or molecule gets slightly charged ends known as a temporary dipole. This charge can then induce an opposite charge in a neighbouring atom or molecule called an induced dipole. The oppositely charged ends attract each other creating the intermolecular force. δ- δ+

The relative strength of the force depends on the size of the atoms or molecules. Dispersion forces increase with increasing atomic and molecular size.

Intermolecular Forces Polar molecules—You have already seen that polar bonds arise when there is a difference in electronegativity between two bonded atoms. This results in the bonding electrons being closer to one atom than the other. The arising dipole is permanent and therefore the arising Van der Waal forces are stronger than the London dispersion forces caused by temporary dipoles. This type of Van der Waal force is sometimes called a permanent dipole/permanent dipole interaction.

Permanent dipole-permanent dipole Attractions A polar molecule is one which has permanently charged ends (permanent dipole). Polar-Polar attractions (permanent dipole-permanent dipole) are the intermolecular force of attraction between the oppositely charged ends of the polar molecules. These forces of attraction between polar molecules are in addition to London Dispersion Forces.

Effect of permanent dipole-permanent dipole interactions Propanone Butane Formula Mass 58 58 Structure Intermolecular forces London London + permanent dipole- permanent dipole Boiling Point 56oC 0oC Polar molecules have higher boiling points than non-polar molecules of a similar mass due to the permanent dipole-permanent dipole interactions. Permanent dipole-permanent dipole interactions are stronger than London Dispersion forces.

Intermolecular Forces

Intermolecular Forces

Hydrogen Bonding Hydrogen bonds are permanent dipole-permanent dipole interactions found between molecules which contain highly polar bonds. They are usually found in molecules where hydrogen is bonded to very electronegative atoms like fluorine, oxygen or nitrogen (+ chlorine). Other examples include ammonia, alkanoic acids and alkanols.

Hydrogen bonds are stronger than permanent dipole-permanent dipole attractions and London Dispersion forces but weaker than covalent bonds. Effects of Hydrogen Bonding When Hydrogen bonds are present, the compound will have a much higher melting point (m.pt) and boiling point (b.pt) than other compounds of similar molecular size. E.g. Ethanol Ether

Tell me three things... you have done well you would like to find out more about you know now that you didn’t know 50 minutes ago

Starter Activity Activity 1.14: Ammonia Fountain Your teacher may demonstrate the “Ammonia Fountain Experiment” to demonstrate how soluble ammonia is in water (both have hydrogen bonds between molecules). Ammonia is polar, and so is water. ‘Like dissolves like’

Lesson 2: Hydrogen Bonding Today we will learn to Explain the dramatic effects hydrogen bonding can have on the properties of substances. We will do this by Doing some experiments on viscosity and density, and watching a ice bomb go off!. We will have succeeded if We can describe at least 3 different types ways hydrogen bonding affects properties.

Hydrogen Bonding Activity 1.13 Viscosity : Compare the viscosity of pentane, ethanol, ethan-1,2-diol and propan-1,2,3-triol . How might we do this? Density : I. Place an ice cube into a small beaker half full of vegetable oil and watch what happens as the ice cube melts. .Your teacher may demonstrate how an ice bomb works.

The Ice Bomb Explain on the whiteboards, using the words ‘density’ and ‘hydrogen bonding’ how this works.

Why do pipes burst when water freezes and why does ice float on water? As matter is cooled, it normally contracts and becomes more dense. However, as water freezes it expands (at about 4oC) because the strong hydrogen bonds between the molecules force them into an open lattice structure. This makes the solid ice less dense (takes more space) than the liquid so ice floats on water and pipes burst when water freezes.

Other Effects of Hydrogen Bonding Viscosity 1.Substance diethyl ether ethanol water glycerol 2.Molecular mass 74 46 18 92 3.Structural Formula 4. No of –OH groups 0 1 2 3 Increasing Viscosity

Viscosity is not only related to molecular mass but also to Hydrogen bonding. The –OH groups allow hydrogen bonding between the molecules and this increases the viscosity. Miscibility Miscible liquids mix thoroughly without any visible boundary between them, e.g. ethanol and water would be described as miscible but water and oil are immiscible as the oil forms a visible layer on water. Hydrogen bonding aids miscibility (ethanol and water both contain hydrogen bonds). NB very strongly polar liquids (without Hydrogen bonding) can also be miscible with water.